High energy, low burning rate solid propellant compositions based on acrylic prepolymer binders

ABSTRACT

High energy, low burning rate solid propellant compositions are disclosed. The disclosed compositions employ acrylic prepolymers selected from methyl acrylate/acrylic acid copolymer, ethyl acrylate/acrylic acid copolymer, 2-ethylhexyl acrylate/acrylic acid copolymer, butyl acrylat/acrylic acid copolymer, petrin acrylate/acrylic acid copolymer, or 2,2-dinitropropyl acrylate/acrylic acid copolymer in combination with plasticizers selected from the group of plasticizers consisting of triethyleneglycol dinitrate (TEGDN), trimethylolethane trinitrate (TMETN), butanetriol trinitrate (BTTN), glycerine trinitrate (NG), 1,2,3-tris 1,2-bis(difluoroamino)ethoxy propane (TVOPA), and blends of the same, curing catalyst, a mixed oxidizer comprised of ammonium perchlorate and cyclotetramethylene tetranitramine (HMX), and aluminum as fuel.

United States Patent [191 Baldwin et al.

[ Apr. 16, 1974 [75] Inventors: Mart G. Baldwin, Newtown, Pa.;

Paul H. Gehlhaus, Huntsville, Ala.

[ 73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: Jan. 7, 1970 [21] Appl. No.: 7,324

Primary ExaminerBenjamin R. Padgett Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; James T. Deaton [57] ABSTRACT High energy, low burning rate solid propellant compositions are disclosed. The disclosed compositions employ acrylic prepolymers selected from methyl acrylate/acrylic acid copolymer, ethyl acrylate/acrylic acid copolymer, 2-ethylhexyl acrylate/acrylic acid copolymer, butyl acrylat/acrylic acid copolymer, petrin acrylate/acrylic acid copolymer, or 2,2-dinitropropyl acrylate/acrylic acid copolymer in combination with plasticizers selected from the group of plasticizers consisting of triethyleneglycol dinitrate (TEGDN), trimethylolethane trinitrate (TMETN), butanetriol trinitrate (BTTN), glycerine trinitrate (NG), 1,2,3-tris 1,2- bis(difluoroamino)ethoxy propane (TVOPA), and blends of the same, curing catalyst, a mixed oxidizer comprised of ammonium perchlorate and cycl0tet ramethylene tetranitramine (HMX), and aluminum as fuel.

13 Claims, No Drawings HIGH ENERGY, LOW BURNING RATE SOLID PROPELLANT COMPOSITIONS BASED ON ACRYLIC PREPOLYMER BINDERS BACKGROUND OF THE INVENTION High energy double base propellants containing both ammonium perchlorate and HMX as oxidizers have demonstrated delivered specific impulse values of 260 sec., with theoretical specific impulse of about 270 seconds. These propellants are difficult to process and generally the mechanical properties and sensitivity characteristics are relatively poor.

Acrylic prepolymer binders have been used in high energy NF propellants containing ammonium perchlorate and aluminum, and in non-aluminized low signature propellants with HMX as the oxidizer. The techniques for prepolymers preparation and characterization and for propellant preparation are well understood.

There exists a need for improved high specific impulse, low burning rate solid propellants for use in a variety of rockets, particularly for upper stages of large rockets.

The requirements of high impulse and low burning rate indicate that a specific impulse of about 270 sec. and a base burning rate (e.g., a burning rate obtained without any added burning rate modifier) of about 0.3 inches/seconds at 1,000 psi would be desirable characteristics for the needed propellant. The propellant should be easily processed and capable of being cured to a solid rubbery product having qualitatively attractive mechanical properties. The base burning rate in the range specified permits an adjustment of the burning rate, if desired to be lowered, by inclusion of known burning rate retarders, and if desired to be raised, by inclusion of small particle size ammonium perchlorate.

An object of this invention is to provide acrylic prepolymers which are compatible with a wide variety of plasticizers and other solid propellant ingredients.

Another object is to provide solid propellants with calculated specific impulse values of about 270 sec., and having burning rates of about 0.3 inches/sec., at 1,000 psi.

A further object is to provide solid propellant compositions which may be readily tailored by means of copolymerization technique whereby small amounts of appropriate comonomers are blended with the acrylic prepolymers of this invention to achieve a desired end product in the cured propellant.

SUMMARY OF THE INVENTION A prepolymer prepared from an acrylate selected from methyl acrylate, ethyl acrylate, 2-ethylhexyl acrylate, butyl acrylate, petrin acrylate, and 2,2- dinitropropyl acrylate, and acrylic acid is used in combination with plasticizers selected from triethyleneglycol dinitrate (TEGDN), trimethylolethane trinitrate (TMETN), butanetriol trinitrate (BTTN), glycerin trinitrate (NG), 1,2,3-tris l,2-bis(difluoramino) ethoxy propane (TVOPA) and blends of the same, aluminum metal fuel, a mixed oxidizer comprised of ammonium perchlorate (AP) and cyclotetramethylene tetranitramine (HMX and a suitable curing agent such as dicyclo diepoxy carboxylate to yield high energy, low burning rate solid propellants. A typical propellant burns at about 0.3 inches/sec. with a pressure exponent of about 0.6 while under 1,000 psi pressure. The theoretical specific impulse is'about 270 sec.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The prepolymers or copolymers used in this invention are prepared as illustrated by the preparative procedure for petrin acrylate/acrylic acid prepolymer given below.

A solution of 95.0 grams of petrin acrylate, 5.0 grams acrylic acid, and 1.0 gram benzoyl peroxide initiator in 300 cc of an inert organic solvent (e.g., ethyl acetate) is prepared and heated under reflux conditions for sixteen hours, during which period complete polymerization occurs. The polymer is recovered (for testing purposes) from the resulting solution as a brittle solid by precipitating into hexane. The product is soluble in commonly used organic solvents (e.g., acetone, diethyl ether, ethyl acetate and the like). The intrinsic viscosity of the polymer in acetone solution at 30 C is 0.13. When the prepolymer is to be used immediately in-a propellant mix, the inert organic solvent may be removed after the plasticizer addition is made as described later herein below.

The copolymers of methyl acrylate/acrylic acid, ethyl acrylate/acrylic acid, 2-ethylhexyl acrylate/acrylic acid, butyl acrylate/acrylic acid and the copolymers of 2,2-dinitropropyl acrylate/acrylic acid may be prepared by substituting butyl acrylate, methyl acrylate, ethyl acrylate, 2-ethylhexyl acrylate, or 2,2- dinitropropyl acrylate for petrin acrylate in the process set forth above.

A typical high energy, low burning rate propellant composition of this invention with a theoretical lsp of 270.7 sec. is set forth below.

High Energy-Low Burning Rate Propellant The propellant mixes of this invention are prepared by blending with the appropriate amount of prepolymer solution formed from the selected acrylate (-96 parts) and acrylic acid (4-10 parts), and benzoyl peroxide initiator in an inert organic solvent (e.g., ethyl acetate), appropriate amounts of TEGDN and TMETN, and the solvent is stripped under vacuum. The viscous prepolymer/plasticizer solution is weighed into a glass resin kettle and the curing agent, catalyst, and solid ingredients are added. The propellant is mixed with a small air motor driven paddle stirrer for 2 hours at 50 C, and cast into appropriate molds, and cured for 40 hours at 60 C. The resulting propellant is a rubbery solid with qualitatively attractive mechanical properties. The burning rate of the propellant at 1,000 psi is 0.3 inches/sec., with a pressure exponent of 0.6.

A high energy propellant ingredient TVOPA may be employed as the plasticizer for the disclosed propellant composition. The preparation of TVOPA may be accomplished by reacting Tris(vinoxy)propane (prepared in accordance with U.S. Pat. No. 2,969,400) with tetrafluorohydrazine. TVOPA contains two high energy difluoramino groups, NF- in each of the three vinoxy groups of the starting compound, tris(vinoxy)propane. The reaction of tetrafluorohydrazine with tris( vinoxy)- propane to form TVOPA is conducted under pressure in the range of 500 mm of mercury up to about 600 psig and temperature range of about to l20 C. The reaction is conducted in the presence of an inert volatile organic solvent, preferably one that is a suitable solvent for both the TVOPA as well as the reactant. Aromatic and aliphatic hydrocarbons, chlorinated hydrocarbons, ethers and ketones may be employed as the solvent. Typical solvents include diethyl ether, dipropyl ether, pentane, hexane, chloroform, carbon tetrachloride, methylene chloride, benzene, toluene, xylene, and acetone.

When TVOPA is employed in the propellant composition, a potentially higher energetic propellant results since the difluoramino groups present therein yield fluorine for oxidation of the metallic fuel in the composition. Since the acrylic prepolymers are compatible with a wide variety of different plasticizers, (e.g., NF containing nitrate ester, and the like), many propellants with desired properties can be prepared by varying the plasticizer selected for blending with the other ingredients.

The plasticizers are used in amounts of about 10 to about 30 weight percent in the propellant composition depending on the concentration of other ingredients and the desired properties required during propellant processing and in the finished propellant. A mixed plasticizer of at least two plasticizers in ratio of about 50/50 or other suitable ratios may be used. The amount used depends on the properties desired or required for the intended use.

Powdered metals (as fuels) such as aluminum, magnesium, titanium, zirconium, and boron are suitable for use in the disclosed propellant compositions in amounts of from about 5 to about 21 weight percent. Aluminum is the preferred metal.

The prepolymer mix of 95/5, selected acrylate to acrylic acid ratio, may vary in the propellant composition from about to about 25 weight percent. Also, the ratio of selected acrylate to acrylic acid in the prepolymer may vary from about 90/10 to about 96/4; however, the 95/5 mix is the preferred mix for use with the plasticizers specified.

The mixed oxidizer AP/HMX, rather than either oxidizer alone, provides maximum specific impulse. The total oxidizer content in amount from about 35 to about 60 weight percent is generally preferred. The ratio of AP to HMX may be varied to provide different burning rates and desired specific impulses. The ratio of AP/HMX may vary from about V2 to Va. A preferred ratio of about 1 part AP to about 2 1% parts HMX is used to provide a burning rate and pressure exponent in the desired range.

In addition to the major components specified, the propellant compositions of this invention may be tailored by employing trace amount of stabilizers, ballistic modifiers curing agents, processing aids (e.g., lecithin, inert organic solvents) as required for specific conditions in processing and desired properties in the finished propellant.

We claim:

1. High energy, low burning rate solid propellant composition comprised of a copolymer selected from the group consisting of methyl acrylate/acrylic acid copolymer, ethyl acrylate/acrylic acid copolymer, 2- ethylhexyl acrylate/acrylic acid copolymer, petrin acrylate/acrylic acid copolymer, butyl acrylate/acrylic acid copolymer, and 2,2-dinitropropyl acrylate/acrylic acid copolymer; a plasticizer selected from the group of plasticizers consisting of triethyleneglycol dinitrate, trimethylolethane trinitrate. butanetriol trinitrate. l,2,3-tris l,2-bis(difluoramino) ethoxy propane, and blends of the same; metal fuel; and a mixed oxidizer comprised of ammonium perchlorate and cyclotetramethylene tetranitramine.

2. The compostion of claim 1 wherein said acrylate is present in an amount of about to 96 parts to 10 to 4 parts of said acrylic acid.

3. The composition of claim 2 wherein said composition is cured with dicyclo diepoxy carboxylate curing agent.

4. The composition of claim 3 wherein said curing agent is present in an amount of about 2 parts per 100 parts of the other combined propellant ingredients.

5. The composition of claim 4 wherein said metal fuel is aluminum and is present in amounts of from about 5 to about 21 weight percent; said selected copolymer is present in amounts from about 10 to about 25 weight percent; said mixed oxidizer is present in amounts from about 35 to about 60 weight percent, said mixed oxidizer containing ammonium perchlorate ratio to cyclotetramethylene tetranitramine of about /2 to said plasticizer is present in amounts from about 10 to about 30 weight percent.

6. The composition of claim 5 wherein said plasticizer contains at least two plasticizers in a ratio of about 50/50.

7. The composition of claim 5 wherein said copolymer is petrin acrylate/acrylic acid in ratio of /5 and is present in said composition of a weight percent of about l0.1; said plasticizer is a mixture of triethyleneglycol dinitrate and trimethylolethane trinitrate in equal amounts of about 10.1 weight percent each of said composition; said mixed oxidizer contains about 19.0 weight percent of said ammonium perchlorate in said composition and about 30.2 weight percent of cyclotetramethylene tetranitramine in said composition; and said aluminum is powdered and is present in said composition in an amount of about 20.5 weight percent.

8. The composition of claim 5 wherein said selected copolymer is comprised of 2,2-dinitropropyl acrylate/acrylic acid.

9. The composition of claim 5 wherein said selected copolymer is comprised of butyl acrylate/acrylic acid.

10. The composition of claim 5 wherein said selected copolymer is comprised of ethyl acrylate/acrylic acid.

11. The composition of claim 5 wherein said selected copolymer is comprised of methyl acrylate/acrylic acid.

12. The composition of claim 5 wherein said selected copolymer is comprised of 2-ethylhexyl acrylate/acrylic acid.

13. The composition of claim 5 wherein said plasticizer is 1,2,3-tris l,2-bis(difluoramino) ethoxy propane. 

2. The compostion of claim 1 wherein said acrylate is present in an amount of about 90 to 96 parts to 10 to 4 parts of said acrylic acid.
 3. The composition of claim 2 wherein said composition is cured with dicyclo diepoxy carboxylate curing agent.
 4. The composition of claim 3 wherein said curing agent is present in an amount of about 2 parts per 100 parts of the other combined propellant ingredients.
 5. The composition of claim 4 wherein said metal fuel is aluminum and is present in amounts of from about 5 to about 21 weight percent; said selected copolymer is present in amounts from about 10 to about 25 weight percent; said mixed oxidizer is present in amounts from about 35 to about 60 weight percent, said mixed oxidizer containing ammonium perchlorate ratio to cyclotetramethylene tetranitramine of about 1/2 to 1/3 ; said plasticizer is present in amounts from about 10 to about 30 weight percent.
 6. The composition of claim 5 wherein said plasticizer contains at least two plasticizers in a ratio of about 50/50.
 7. The composition of claim 5 wherein said copolymer is petrin acrylate/acrylic acid in ratio of 95/5 and is present in said composition of a weight percent of about 10.1; said plasticizer is a mixture of triethyleneglycol dinitrate and trimethylolethane trinitrate in equal amounts of about 10.1 weight percent each of said composition; said mixed oxidizer contains about 19.0 weight percent of said ammonium perchlorate in said composition and about 30.2 weight percent of cyclotetramethylene tetranitramine in said composition; and said aluminum is powdered and is present in said composition in an amount of about 20.5 weight percent.
 8. The composition of claim 5 wherein said selected copolymer is comprised of 2,2-dinitropropyl acrylate/acrylic acid.
 9. The composition of claim 5 wherein said selected copolymer is comprised of butyl acrylate/acrylic acid.
 10. The composition of claim 5 wherein said selected copolymer is comprised of ethyl acrylate/acrylic acid.
 11. The composition of claim 5 wherein said selected copolymer is comprised of methyl acrylate/acrylic acid.
 12. The composition of claim 5 wherein said selected copolymer is comprised of 2-ethylhexyl acrylate/acrylic acid.
 13. The composition of claim 5 wherein said plasticizer is 1,2, 3-tris 1,2-bis(difluoramino) ethoxy propane. 